net::ERR_EMPTY_RESPONSE means the browser did not receive a usable HTTP response. It does not prove that cy.intercept() caused the failure. The reliable fix is to identify which layer ended the exchange—your route handler, the origin server, a proxy, malformed headers, the browser connection, or a long-lived stream—then repair that layer.
Before changing Cypress configuration, record the Cypress package and binary versions, browser and operating-system versions, headed or headless mode, request URL and method, route matcher and handler, Command Log details, browser network information, and relevant application or proxy logs.
What ERR_EMPTY_RESPONSE tells you—and what it does not
Chrome reports net::ERR_EMPTY_RESPONSE when a connection closes or otherwise produces no usable response. The message is a symptom, not a root-cause diagnosis. A request displayed as intercepted may still fail before a stub replies, while a real upstream request can fail inside Cypress’s proxy even when the route definition is correct.
Start by deciding what the request is supposed to do:
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- Stubbed request: Cypress should terminate the request with a fixture or a handler-generated response.
- Passthrough request: Cypress should observe or modify the request and let the real server answer.
That distinction determines the next checks. The cy.intercept() API supports matching, waiting, modifying, and stubbing, but the error text alone cannot identify which operation failed.
1. Capture a reproducible baseline
- Run
npx cypress versionand save both the npm package and binary versions. Record the browser name and exact version, operating system, and whether the test runs headed or headless. - Copy the failing method and URL exactly, including query parameters. Note whether application code or the test itself initiated the request.
- Save the route matcher and handler, for example
cy.intercept('GET', '**/api/orders', handler), including any asynchronous work, header edits, aliases, andreq.continue()orreq.reply()calls. - Open the Cypress Command Log and browser developer-tools network entry. Record whether Cypress matched the route, whether a request was sent upstream, timing, status, and response headers.
- Collect application-server, reverse-proxy, container, and TLS logs for the same timestamp. These often distinguish an origin reset from a test-runner problem.
Version matters. Cypress 16.0.0 documents native browser network interception for Chrome, Chromium, and Edge and includes interception and waiting changes. Read the version-specific notes in the Cypress changelog before applying advice written for an older release.
2. Verify the route matcher and handler
Match the intended request
A route can silently miss because the method, hostname, pathname, query string, or glob pattern differs from the actual request. Temporarily use an explicit matcher and log the request:
cy.intercept({ method: 'GET', pathname: '/api/orders' }, (req) => {
console.log('matched', req.method, req.url)
req.reply({ statusCode: 200, body: { orders: [] } })
}).as('orders')
cy.visit('/orders')
cy.wait('@orders').its('response.statusCode').should('eq', 200)Use pathname when query parameters vary, or include the exact query when it is significant. Confirm that the intercept is registered before cy.visit() or the action that triggers the request; otherwise the request may occur before the route exists.
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Make completion explicit
A handler that never calls req.reply(), req.continue(), or returns a valid static response can leave the exchange unresolved. Keep the first diagnostic handler simple, then add custom logic back one piece at a time:
cy.intercept('POST', '**/api/login', (req) => {
req.reply({
statusCode: 200,
headers: { 'content-type': 'application/json' },
body: { token: 'test-token' }
})
})Remove temporary header mutations, asynchronous calls, and body transformations. If the simple stub works, reintroduce each change separately. Cypress has recorded a fix for empty-string request or response header handling in an intercept handler; if your route edits headers, verify that your installed version includes the relevant fix and avoid emitting invalid values.
Check aliases and waits
An alias wait timeout is different from an empty response. Confirm that the request actually appears in the Command Log and that the alias is attached to the matching route. Do not “fix” an empty response by increasing responseTimeout until you know the request is merely slow rather than receiving no response.
3. Test the real origin outside Cypress
If the route is intended to pass through, reproduce the same method, URL, headers, cookies, and body with a normal browser or an HTTP client. A direct request that also fails points to the origin, DNS, TLS, or proxy. A direct request that succeeds narrows attention to Cypress’s browser connection, proxy path, or route handler.
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Inspect the server and proxy path
- Verify that the API is listening on the host and port used by the browser inside the Cypress environment.
localhostin a container is not necessarily the host machine. - Check DNS resolution, IPv4 versus IPv6 binding, TLS certificates, and whether a reverse proxy closes idle or oversized connections.
- Correlate the Cypress timestamp with server access and error logs. No server log entry usually means the request never reached the origin.
- Review
HTTP_PROXY,HTTPS_PROXY, andNO_PROXYsettings inherited by the test process and CI runner.
Cypress issue #5895 describes a localhost API that worked outside Cypress but produced this browser error inside the runner. A reporter proposed HTTP_PROXY=localhost, but also described a listener warning and Cypress closure; it is an environment-specific workaround, not a general fix. Do not add it blindly.
4. Inspect raw response headers for proxy failures
If Cypress logs show an internal proxy exception, inspect the exact upstream headers rather than changing route waits. Header names must be valid HTTP tokens. Issue #28865 pairs ERR_EMPTY_RESPONSE with an invalid header named expires (note the trailing space) and ERR_INVALID_HTTP_TOKEN. The repair belongs in the server or intermediary emitting that header: remove the whitespace or correct the proxy configuration.
Also check duplicate or conflicting Content-Length and Transfer-Encoding, truncated compressed bodies, premature connection closes, and non-ASCII characters in header names. Capture headers at the origin and at the proxy when possible; a proxy may rewrite a valid response into an invalid one.
5. Treat retries as a limited diagnostic tool
Cypress’s 2019 article about Cypress 3.3.0 describes retrying some network failures and the retryOnNetworkFailure option for cy.visit() and cy.request(). That historical behavior and its “300%” network-speed headline do not establish current behavior or guarantee that an empty response will be repaired. Check the API for your installed release before using retry options.
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A retry is useful only when logs show a transient connection reset or startup race. It cannot make a persistently broken server, invalid header, or unmatched stub succeed. Prefer waiting for your application to become ready and fixing the first failing layer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Debug EventSource and SSE separately
Server-sent events are not ordinary finite request/response exchanges. The initial connection remains open while the server sends a stream, so browser and proxy idle timeouts, buffering, compression, and connection handling matter.
Separate the request that creates backend data from the long-lived stream that delivers updates. Cypress discussion #29537 describes the trigger request succeeding while the EventSource stream failed with ERR_EMPTY_RESPONSE. Check stream-server logs, Content-Type: text/event-stream, Cache-Control: no-cache, proxy buffering settings, and keep-alive behavior.
Use a deterministic fake for UI tests
For a component test that verifies rendering and reconnection logic, replace EventSource before application code runs:
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cy.visit('/dashboard', {
onBeforeLoad(win) {
class FakeEventSource {
constructor() { this.onmessage = null; this.onerror = null }
close() {}
emit(data) { this.onmessage?.({ data: JSON.stringify(data) }) }
}
win.EventSource = FakeEventSource
}
})cy.visit()‘s onBeforeLoad hook is documented at the Cypress API page. A fake proves only that the UI responds to events. Keep a separate integration test against the real SSE service to validate the stream itself; the community discussion is advice, not a formal support guarantee.
7. A layer-by-layer decision table
| Observation | Likely layer | Next action |
|---|---|---|
| Route never appears in Command Log | Matcher or registration | Register earlier; verify method, URL, origin, and alias. |
| Stub handler is entered but no response is recorded | Handler | Use a minimal req.reply(); remove async work and header edits. |
| Direct client also fails | Origin, DNS, TLS, or proxy | Read server/proxy logs and test host resolution and certificates. |
| Only Cypress fails; proxy error names a header | Malformed upstream response | Correct the emitting server or intermediary. |
| Trigger succeeds but EventSource fails | Streaming protocol or idle timeout | Inspect stream headers and buffering; test UI with a fake and stream separately. |
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Common failed fixes
- Increasing timeouts: helps only when a valid response is slow; it does not repair an empty connection.
- Adding retries everywhere: can hide a deterministic server or matcher defect and lengthen CI runs.
- Changing unrelated route waits: cannot correct malformed headers or proxy failures.
- Forcing
HTTP_PROXY=localhost: is a report-specific workaround with documented side effects. - Mocking SSE and declaring victory: validates UI behavior, not the real stream.
What a useful bug report contains
Include a minimal test, Cypress package and binary versions, browser and OS, headed or headless mode, exact method and URL, matcher and handler, Command Log and network export, direct-client comparison, raw response headers, proxy variables, and synchronized origin logs. State whether the request should be stubbed, passed through, or streamed. This lets maintainers reproduce the failing layer instead of guessing from the browser error alone.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsFrequently Asked Questions
Does ERR_EMPTY_RESPONSE prove cy.intercept() canceled the request?
No. It only reports that the browser received no usable response; the origin, proxy, malformed headers, browser connection, route handler, or stream can be responsible.
Should I use a fake EventSource in every Cypress test?
No. Use it for deterministic UI tests, and keep a separate integration test for the real SSE endpoint.
When is a retry appropriate?
Only after logs show a transient network failure and your installed Cypress version supports the option. Retries do not repair persistent server or proxy defects.
Quick Recap
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